Patient populations are studied to clarify the long-term effects of exposure to medical irradiation, cytotoxic agents and other therapeutic drugs. Multiple primary cancers are studied to evaluate late effects of therapy and to generate hypotheses about common etiologies. Patients genetically predisposed to cancer are studied for insights into mechanisms of carcinogenesis. Principal findings were as follows: (1) Solid cancer risk increased over time following bone marrow transplantation, and was higher for patients treated as children; (2) an excess of bone and soft tissue sarcoma among bilateral retinoblastoma patients is ascribable to high-dose radiation; (3) benign thyroid nodules, but not thyroid cancer, was linked to diagnostic doses of radioactive iodine (I-131); (4) total cancer mortality was not elevated 5 years or more following treatment for hyperthyroidism; however,suggestive associations were seen between thyroid cancer mortality and I-131 treatment and between overall cancer mortality and anti-thyroid drug treatment; (5) salivary gland tumors were linked to childhood radiotherapy for enlarged tonsils; (6) the success of radiotherapy for cervical cancer has resulted in large numbers of long term survivors at increased risk of second cancers late in life; (7) cumulative risks were 2.5% for malignant mesothelioma and 11% for lung cancer following exposure to radioactive Thorotrast; (8) a 5-fold lung cancer excess was found among 10-year survivors of radiotherapy for Hodgkin's disease, with a disproportionate number of G:C to A:T transitions in the p53 tumor suppressor gene; breast cancer risk was significantly increased among patients treated before age 16; (9) there was a significant risk of secondary leukemia following low-dose total body irradiation and chemotherapy for non-Hodgkin's lymphoma; (10) significant excesses of ocular melanoma and acute lymphoblastic leukemia were found among ovarian cancer patients; (11) cyclophosphamide-related bladder cancer was found to be dose-dependent; (12) tamoxifen therapy for breast cancer was linked to a 2-fold risk of endometrial cancer; (13) non-Hodgkin's lymphoma was increased among patients prescribed amphetamines; (14) childhood cancer was not predictive of cancer in a parent, however, osteosarcoma before age 3 was predictive of early onset breast cancer in mothers; (15) endometrial carcinoma was significantly elevated among infertility patients, but ovulation-stimulating drugs were not significantly linked to ovarian carcinoma; (16) a joint analysis of 13 studies found a 10-fold risk of thyroid cancer associated with a history of benign thyroid disease.